1. Field of the Invention
The invention relates to a polarized light irradiation device for purposes of optical alignment of an alignment layer of a liquid crystal cell element by irradiation with polarized light. The invention relates especially to a polarization light irradiation device for purposes of optical alignment of an alignment layer in which irradiation of a large workpiece with polarized light can be achieved.
2. Description of Related Art
The liquid crystal cell element usually consists of two substrates. On one substrate, a driver system for triggering the liquid crystal (for example, a thin film transistor) and electrodes for triggering the liquid crystals which are composed of transparent conductive films, and an alignment layer or the like for aligning the liquid crystal in a certain direction are formed. On the other of these substrates, a light screening film, which is called a black matrix, is formed. In a color liquid crystal cell element, a color filter and the above described alignment layer are formed.
The alignment layer is usually produced by subjecting the surface of a thin film, such as polyamide resin or the like, to a treatment which is called rubbing, and providing it with grooves in a certain direction; this is designed to align the molecules of the liquid crystals along these fine grooves in a certain direction. In the above described rubbing treatment, a process is widely used in which the alignment layer is produced by rubbing the substrate with a cloth wound around a rotating roller.
The above described formation of the alignment layer is accomplished by rubbing the substrate with the cloth as the rubbing medium. Therefore, here, adverse effects, such as dust, static electricity, scratches and the like cannot be avoided, nor can a decrease of yield.
Recently, therefore, a technique was proposed in which the alignment of the liquid crystals is produced without the above described rubbing (the alignment technique without rubbing is hereinafter called "nonrubbing").
In this nonrubbing technique, there is a process using polarization. In this process, the thin film, such as polyamide or the like, is irradiated with polarized light, the polymer of the thin film is subjected to chemical conversion in a certain direction and thus alignment is accomplished.
To use this technique to form the alignment layer of the liquid crystal substrate, however, a large polarized light irradiation device is necessary. This means that, in the case of producing the liquid crystal cell elements conventionally, four to six liquid crystal cell elements are formed on the substrate. The size of the substrate as the object of light irradiation is therefore conventionally roughly 550 mm.times.650 mm. Therefore, an area of roughly 800.times.800 mm is needed; it is irradiated by the polarized light irradiation device which is used to form the alignment layers of the liquid crystal substrates. To date there has not been any polarized light irradiation device which irradiates such a large area.
FIG. 8 schematically shows the arrangement of a light irradiation device for emitting ultraviolet rays as the prerequisite of the invention. The light irradiation device conventionally comprises, as shown in the drawing, a discharge lamp 1, such as super high pressure mercury lamp 1, an oval focusing mirror 2, a first flat mirror 3, an integrator lens 5, a shutter 4, a second flat mirror 6 and a collimation lens 7. Light which contains the ultraviolet light emitted from discharge lamp 1 is focused by oval focusing mirror 2, is reflected by first flat mirror 3, and is incident on integrator lens 5. The light emerging from integrator lens 5 is reflected via shutter 4 and then by second flat mirror 6, is converted into parallel light by means of collimation lens 7, and emerges from the light irradiation device. The parallel light emerging from the light irradiation device is emitted via mask M onto a workpiece W, such as a liquid crystal substrate or the like. Furthermore, to irradiate workpiece W with light via mask M, on which a stipulated pattern is formed, and to expose only one predetermined position of the workpiece W, it is necessary to obtain parallel light by means of collimation lens 7, and thus, to irradiate mask M and workpiece W as is described above.
Furthermore, reference number 10 designates an alignment microscope by which alignment marks of mask M and alignment marks of workpiece W are observed, and by which the mask M is aligned with respect to workpiece W. Afterwards, ultraviolet light is emitted from the above described light irradiation device and directed through mask M. Furthermore, instead of the above described second flat mirror and collimation lens 7, collimation mirrors can be used which consist of concave mirrors.
The emergence of polarized light from the above described light irradiation device is enabled by the arrangement of a polarization element which is located in the optical path between oval focusing mirror 2 and mask M, and which polarizes the light. Here, however, depending on the location at which the polarization element is located, the following disadvantages arise:.
(1) If there is a polarization element in front of collimation lens 7 where there is still no parallel light, some of the polarized light irradiating workpiece W is deflected to one polarization direction on the irradiated surface on the optical axis with a certain angle. This means that, in the manner shown in FIG. 9, a polarization device according to arrow B in crosshatched area is tilted, for example, to one polarization direction on an irradiated surface around the optical axis according to arrow A. PA0 (2) If instead of above described second flat mirror 6 and collimation lens 7, metallic collimation mirrors are used, or if a metallic mirror is used as a second flat mirror 6, and if the polarization element is placed in front, the light with which workpiece W is irradiated is converted into an oval polarization, causing the same defects as described above. PA0 (3) If at the output of integrator lens 5 with high illumination intensity the polarization element is arranged, the polarization element is more frequently degraded by the UV light. PA0 (1) For optical alignment of the alignment layer of the liquid crystal cell element, it is necessary to fix the ratio of the S-polarized light relative to the P-polarized light which is emitted onto the workpiece, that is, S/P, at less than or equal to a stipulated value. PA0 (2) In the light irradiation device with the arrangement shown in FIG. 8, the degree of parallelism of the central light beam is conventionally .+-.1.degree. and the angle of vision is roughly .+-.2.degree.. Furthermore, an error of roughly .+-.2.degree. of the installation angle in the slanted arrangement of the glass plates is expected. Therefore, it is possible that light inclined by roughly .+-.5.degree. relative to the Brewster angle is incident on the glass plates. It is therefore necessary that, with regard to the light inclined by .+-.5.degree. with respect to the above described Brewster angle, the attenuation coefficient of the light at the polarization element is less than or equal to a stipulated value in order to effect optical alignment of the alignment layer of the liquid crystal cell element with high efficiency. PA0 (1) In a light irradiation device which has a lamp which emits light which contains UV light, a focusing mirror which focuses the light from the lamp, an integrator lens and a collimation lens or collimation mirror, on the exit side of the collimation lens or the collimation mirror there is a polarization element in which there are several glass plates which are parallel to one another at a distance and inclined by the Brewster angle to the primary light beam, the number of glass plates of the polarization element being fixed such that the ratio of the component of the vertical polarized light S to the component of parallel polarized light P, i.e., ratio S/P, is less than or equal to 0.1, and that attenuation of the light with parallel polarization P with reference to the incident light with an inclination of .+-.5.degree. to the above described Brewster angle is less than or equal to 1/2. PA0 (2) The above described glass plates are formed from quartz glass.
If the substrate is irradiated with this light which has polarization directions which are not uniform as described above, the direction in which the alignment layer is aligned is no longer uniform. The images obtained by the liquid crystal cell element which is provided with the alignment layer with nonuniform alignment directions exhibit scattering of the shade and contrast, depending on the pertinent locations.
Therefore, there is a requirement for uniformity of the polarization directions of the polarized light with which the substrate is irradiated. It is necessary to arrange the polarization element between collimation lens 7 and mask M, where the light is parallel. Furthermore, in the case of using the collimation mirror it is desirable to arrange the polarization element between the collimation mirror and mask M.
As was described above, the problem of nonuniformity of the polarization directions or the like occurs when the polarization element is located in front of collimation lens 7. It is therefore necessary to place the polarization element between collimation lens 7 and mask M in order to allow the polarized light to emerge from the light irradiation device shown in FIG. 8.
But, in the case in which the polarization element is located between collimation lens 7 and mask M, a polarization element with a size which is essentially equal to or larger than the size of workpiece W must be used.
The following are known as large polarization elements in this sense:
(1) a polarization element using resin (polymer); and PA1 (2) a polarization element in which glass or a plastic film is provided with a vacuum evaporated film.
The polarization element according to (1) is often degraded by the UV light. The polarization element according to (2) has a narrow wave band in which polarization is possible. Furthermore, it is considered disadvantageous that, for a low incline of the polarization element, the wave band in which polarization is possible is shifted, and that, in this way, calibration during installation into the above described light irradiation device is difficult. In addition, there is the possibility of loosening or degradation of the vacuum evaporated film, making maintenance difficult.